Size Separation Large pieces of material are usually estimated visually, difficulties arising only in the estimation of powders. Standards for Powders Standards for powders for pharmaceutical purposes are laid down principally in the British Pharmacopoeia which states, that the degree of coarseness or fineness of a powder is differentiated and expressed by the size of the mesh of the sieve through which the powder is able to pass.
Size Separation The BP specifies five grades of powder and the number of the sieve through which all the particles must pass. Grade of powder Sieve through which all particles must pass Coarse 10 Moderately coarse 22 Moderately fine 44 Fine 85 Very fine 120
The BP specifies a second, smaller size of sieve for the coarser powders but states the not more than 40 per cent shall pass through. The relevant grades of powder and sieve number are shown in the table: Sieve through which Sieve through which all particles must not more than 40 per Grade of powder pass cent of particles pass Coarse 10 44 Moderately coarse 22 60 Moderately fine 44 85 Fine 85 Not specified Very fine 120 Not specified
Thus, the full definition of Coarse Powder is that : It is powder all the particles of which pass through a No. 10 sieve and not more than 40 percent through a No. 44 sieve, this is usually referred to as a 10/44 powder. Other grades are expressed in a similar way.
• The British Pharmacopoeia makes two statements with regard to these `official' grades of powders in practice: 1. It is required that, when a powder is described by a number, all particles must pass through the specified sieve. 2. When a vegetable drug is being ground and sifted, none must be rejected. • The reason for this will be apparent if the character of a vegetable drug is compared with a chemical substance. The latter is a homogeneous material so that, if a certain quantity, of a powder is required, an excess may be ground, a sufficient amount of the desired size range obtained by sieving, and the oversize particles (known as tailings) may be discarded.
A vegetable drug, however, consists of a variety of tissues of different degrees of hardness, so that softer tissues will be ground first and tailings obtained by sifting will contain a higher proportion of the harder tissues. In many cases, constituents are not distributed uniformly through vegetable tissues; for example, in digitalis the glycosides are concentrated in the mid-rib and veins. Hence, if tailings are discarded when grinding and sifting the drug, it is likely that a high proportion of the active constituents will be lost.
In addition to the grades of powder specified by the British Pharmacopoeia, the British Pharmaceutical Codex details a further grade known as Ultra-fine Powder. In this case, it is required that the maximum dimension of at least 90 percent of the particles must be not greater than 5 µm and none must be greater than 50 µm. Determination of particle size for this grade is carried out by a microscopic method.
Sie Sieve ves Sieves for test purposes are the subject of a British Standard. Most of the sieves used are of the wire mesh type , the number of the sieve indicating the number of meshes included in a length of 25.4 mm (1 inch) in each direction parallel to the wires .
It should be noted that it is the number of meshes that is specified and not the number of wires. Thus, a No. 10 sieve has 10 meshes per inch in each direction, but it will be realized that if there were 10 wires there would be 9 meshes only. The simple statement of the number of meshes per unit length is not sufficient, however, as the size of the particle that will pass the sieve will depend on other factors, principally the diameter of the wire
Effect of wire diameter on sieve mesh size.
STANDARDS FOR SIEVES according to B.P. It is required that wire-mesh sieves shall be made from wire of uniform, circular cross-section and for each sieve the following particulars are stated: eve Numbe ber of of Siev This is the number of meshes in a length of 25.4 mm (1 in.), in each direction, parallel to the wires.
Nominal nal Size ze of of Ap Aper erture re (h (hole) This is the distance between the wires, so that it represents the length of the side of the square aperture. Nominal Diameter of the Wire This dimension and the number of meshes form the basic standards for the sieve. The wire diameter has been selected to give a suitable aperture size, but also to have sufficient strength to avoid distortion.
Approximate Screening Area. This standard expresses the area of the meshes as a percentage of the total area of the sieve. It is governed by the size of wire used for any particular sieve number and is kept within the range 35 to 40 per cent. This gives suitable strength to the sieve, but leaves adequate area of meshes since these are obviously the useful area of the sieve.
Ap Aper erture ture To Tolera erance nce Av Aver erage age Some variation in the aperture size is unavoidable and this variation, expressed as a percentage, is known as the aperture tolerance average. The term tolerance is used in engineering practice to mean the limits within which a particular quantity or dimension can be allowed to vary and still be acceptable for the purpose for which it is required.
Finer wires are likely to be subject to a greater proportional variation in diameter than coarse. fine meshes cannot be woven with the same accuracy as coarse meshes. Hence, the aperture tolerance average is smaller for sieves of 5 or 10 mesh than is the case for 300 mesh.
PERFORATED PLATE SIEVES Sieves may also be made by drilling holes in metal plate, so that this type will have circular apertures as against the square apertures of the wire mesh sieve. In general, these sieves are used in the larger sizes and can be made with greater accuracy than wire-mesh sieves, as well as being less susceptible to distortion in use. This type is commonly used also as screens in impact mills.
Usually, the holes are spaced with their centers arranged at the apices of equilateral triangles, so that all the apertures are equidistant A Perforated plate sieve.
Similar standards are laid down with the appropriate equivalent specifications for plate thickness and nominal width of the bridge (dimension A in the Figure) which control the strength of the sieve in the same way as wire diameter in wire mesh sieves.
Ma Material als s Use sed d fo for Si Sieve ves 1) The wire should be of uniform, circular cross-section. 2) The material should have suitable strength to avoid distortion 3) Be resistant to corrosion by any substances that may be sifted.
METALS Iron wire Advantage cheap, Disadvantage • Rusting • Iron contamination of products
METALS Coated Iron ( coating with galvanizing or tinning). Advantage • Increases the protection against corrosion • Increases the strength Disadvantage Coating after manufacture lead to some variation in the mesh size.
METALS Copper Advantage Avoiding the risk of iron contamination Disadvantage As a soft metal, meshes can be distorted easily.
METALS Copper Alloys ( brass and phosphor-bronze) Advantages • Resemble copper in possessing good resistance to corrosion • Their strength is greater so that less risk of the meshes distortion.
METALS Stainless Steel Advantages • Good resistance to corrosion • Adequate strength • The most suitable for pharmaceutical purposes. Disadvantages Expensive
NON-METALS Used when all risk of metallic contamination be avoided. Used for sieves with fine meshes, since non-metal fibers are stronger than a metal wire of similar thickness.
NON-METALS Materials of natural origin (hair and silk), are used but synthetic fibers (nylon and terylene) are more suitable Advantages of synthetic fibers • Have more strength and resistance to corrosion. • can be extruded in all diameters, so enabling a wide variety of sieves to be made.
Sie ieving ving Me Metho thods ds Sieves should be used and stored with care, since a sieve is of little value if the meshes become damaged or distorted. With the exception of the use of sieves for granulation, material should never be forced through a sieve. Particles, if small enough, will pass through a sieve easily if it is shaken, tapped, or brushed.
I. MECHANICAL SIEVING METHODS Principle: Based on methods as: Agitation Brush the sieve Use centrifugal force
1. Agitation Methods Sieves may be agitated in a number of different ways: Oscillation ( move back and forth) The sieve is mounted in a frame that oscillating. Advantages Simple method Disadvantages The material may roll on the surface of the sieve, and fibrous materials tend to “ ball ”.
Vibration The mesh is vibrated at high speed, often by an electrical device. Advantages The rapid vibration is imparted to the particles on the sieve and the particles are less likely to “blind” the mesh.
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